Potential for Deep Basin-Centered Gas

Accumulation in Hanna Basin, Wyoming

By Michael S. Wilson, Thaddeus S. Dyman, and Vito F. Nuccio

U.S. Geological Survey Bulletin 2184-A

U.S. Department of the Interior

U.S. Geological Survey

Geologic Studies of Basin-Centered Gas Systems

Edited by Vito F. Nuccio and Thaddeus S. Dyman

This work funded by the U.S. Department of Energy, National 

Energy Technology Laboratory, Morgantown, W. Va., under 

contracts DE-AT26-98FT40031 and DE-AT26-98FT40032, and by the 

U.S. Geological Survey Central Region Energy Resources Team
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

Any use of trade, product, or fi rm names in this publication

is for descriptive purposes only and does not

imply endorsement by the U.S. Government

This publication is only available online at:

http://geology.cr.usgs.gov/pub/bulletins/b2184-a/

Posted online June 2001, version 1.0
III

Contents

Abstract.......................................................................................................................................................... 1

Introduction ................................................................................................................................................... 1

Continuous-Type Accumulations ............................................................................................................... 1

Basin-Centered Gas Accumulations ................................................................................................ 2

Assessment of Basin-Centered Gas Accumulations.............................................................................. 3

Geologic Setting of Hanna Basin ............................................................................................................... 3

Exploratory Drilling ....................................................................................................................................... 5

Evidence for Normal Pressures ................................................................................................................. 5

Evidence for Subnormal Pressures ........................................................................................................... 5

Evidence for Overpressured Zones ......................................................................................................... 10

Discussion: Basin-Centered Gas in the Deep Hanna Basin?.............................................................. 10

Conclusions ................................................................................................................................................. 11

References Cited ........................................................................................................................................ 11

Figures

 1. Map showing location of the Hanna Basin, surrounding uplifts, 

major faults, depth in feet to Precambrian basement in the basin center, 

and location of the Brinkerhoff Hanna Unit No. 1 well........................................................... 2

 2. Stratigraphic chart showing names and average thicknesses of 

stratigraphic units recognized in the Hanna Basin, Wyoming.............................................. 4

 3. Map showing structural elements, outcrop patterns, cross sections, 

and locations of 29 representative exploration wells in the 

Hanna Basin, Wyoming ............................................................................................................... 6

 4. Cross section A-A’ showing generalized structure of the Hanna Basin 

and approximate boundaries of the normal, subnormal, and 

overpressure zones based on interpretations of vitrinite reflectance 

measurements and well data ..................................................................................................... 7

 5. Cross section B-B’ showing generalized structure of the Hanna Basin 

and interpreted boundaries of normal, subnormal, and overpressure zones 

based on interpretations of vitrinite reflectance measurements and well data ............... 8

Table

 1. Mud weights, bottom-hole temperatures and drill-stem test results for 29 

representative exploration wells in the Hanna Basin, Wyoming, based on 

well logs and scout cards available at the Denver Earth Resources Library, 

Denver, Colo................................................................................................................................... 9
Abstract

The potential for a continuous-type basin-centered gas 

accumulation in the Hanna Basin in Carbon County, Wyoming, is evaluated using geologic and production data including mud-weight, hydrocarbon-show, formation-test, bottomhole-temperature, and vitrinite refl ectance data from 29 exploratory wells. 

This limited data set supports the presence of a hypothetical basin-centered gas play in the Hanna Basin. Two generalized structural cross sections illustrate our interpretations 

of possible abnormally pressured compartments. Data indicate 

that a gas-charged, overpressured interval may occur within the 

Cretaceous Mowry, Frontier, and Niobrara Formations at depths 

below 10,000 ft along the southern and western margins of 

the basin. Overpressuring may also occur near the basin center 

within the Steele Shale and lower Mesaverde Group section at 

depths below 18,000 to 20,000 ft. However, the deepest wells 

drilled to date (12,000 to 15,300 ft) have not encountered overpressure in the basin center. This overpressured zone is likely to 

be relatively small (probably 20 to 25 miles in diameter) and is 

probably depleted of gas near major basement reverse faults and 

outcrops where gas may have escaped. Water may have invaded 

reservoirs through outcrops and fracture zones along the basin 

margins, creating an extensive normally pressured zone. 

A zone of subnormal pressure also may exist below the 

water-saturated, normal-pressure zone and above the central 

zone of overpressure. Subnormal pressures have been interpreted in the center of the Hanna Basin at depths ranging 

from 10,000 to 25,000 ft based on indirect evidence including 

lost-circulation zones. Three wells on the south side of the 

basin, where the top of the subnormally pressured zone is 

interpreted to cut across stratigraphic boundaries, tested the 

Niobrara Formation and recovered gas and oil shows with very 

low shut-in pressures. 

Introduction

The primary purpose of this report is to describe the 

potential for a continuous-type basin-centered gas accumulation in the Hanna Basin of south-central Wyoming (fi g. 1) 

using the published literature and computerized well and reservoir data fi les. The U.S. Geological Survey (USGS) is currently reevaluating the potential for basin-centered gas accumulations in high-priority basins in the United States in order 

to accommodate changing geologic perceptions and new data 

since the completion of the U.S. Geological Survey 1995 

National Petroleum Assessment. This effort, which is being 

conducted with funding from the U.S. Department of Energy, 

may result in the identifi cation of new continuous-type assessment units and petroleum systems. These potential basincentered gas accumulations vary qualitatively from low to high 

risk and may or may not survive rigorous geologic scrutiny 

leading toward a full geologic assessment based on assessment 

units and petroleum systems.

This report on the Hanna Basin is one of several reports 

that will be published as U.S. Geological Survey Bulletins. 

Data relating to the existence of basin-centered gas accumulations is summarized in each of these reports. No attempt 

is made, however, to identify potential assessment units and 

petroleum systems or to actually assess gas resources for 

potential assessment units. These reports are specifi cally meant 

to describe the geologic and production characteristics of 

potential assessment units.

This work is funded by the U.S. Department of 

Energy, National Energy Technology Laboratory, Morgantown, 

W. Va. (contract nos. DE-AT26-98FT40031 and DE-AT26-

98FT40032), and by the U.S. Geological Survey Central 

Region Energy Resources Team. We wish to acknowledge the 

helpful reviews of Mark Kirschbaum, Ronald Johnson, Thomas 

Judkins, and Katharine Varnes of the U.S. Geological Survey.

Continuous-Type Accumulations

Continuous-type accumulations are large single fi elds 

having spatial dimensions equal to or exceeding those of conPotential for Deep Basin-Centered Gas 

Accumulation in Hanna Basin, Wyoming

By Michael S. Wilson,1 Thaddeus S. Dyman,2 and Vito F. Nuccio2

1

 Geologic consultant, Denver, Colo.

2

 U.S. Geological Survey, Denver, Colo.

1
ventional fi elds. They cannot be represented in terms of discrete, countable units delineated by downdip hydrocarbonwater contacts (as are conventional fi elds). The defi nition of 

continuous-type accumulations used here is based on geology 

rather than on government regulations defi ning low-permeability (tight) gas. Common geologic and production characteristics of continuous-type accumulations include their occurrence 

downdip from water-saturated rocks, lack of obvious traps or 

seals, relatively low matrix permeability, abnormal pressures, 

large in-place hydrocarbon volumes, and low recovery factors 

(Schmoker, 1996).

Continuous-type plays were treated as a separate category 

in the U.S. Geological Survey 1995 National Petroleum 

Assessment and were assessed using a specialized methodology (Schmoker, 1996). These continuous-type plays are geologically diverse and fall into several categories including coalbed gas, biogenic gas, fractured-shale gas, and basin-centered 

gas accumulations. This report focuses on basin-centered gas.

Basin-Centered Gas Accumulations

Basin-centered gas accumulations form a special group 

of continuous-type gas accumulations and differ signifi cantly 

Figure 1. Map showing the location of the Hanna Basin (dark shading), surrounding uplifts (light shading), major 

faults, depth in feet to Precambrian basement in the basin center, and location of the Brinkerhoff Hanna Unit No. 1 

well (BKH). Modifi ed from Perry and Flores (1997, p. 52). 

WYOMING

107 106 30' 106

42

41 30'

42 30'

RAWLINS

RAWLINS

UPLIFT

SIMPSON

RIDGE

HANNA

BASIN - 30,000

- 10,000

PARK

RANGE

GRANITE MOUNTAINS

SHIRLEY

 MOUNTAINS

BKH

SEMINOE MTNS

MEDICINE

BOW

UPLIFT

Thrust fault—

sawteeth on

upper plate

CARBON BASIN

0 10 20 

MILES

SHIRLEY

BASIN

ELK MOUNTAIN

2 Geologic Studies of Basin-Centered Gas Systems
in their geologic and production characteristics from conventional accumulations. They have the following characteristics:

 1. They are geographically large and cover from tens 

to hundreds of square miles in areal extent, typically 

occupying the central, deeper parts of sedimentary 

basins. 

 2. They lack downdip water contacts, and thus hydrocarbons are not held in place by the buoyancy of water.

 3. Reservoirs are abnormally pressured.

 4. Gas is the pressuring phase of the reservoir.

 5. Water production is usually low or absent, or water 

production is not associated with a distinct gas-water 

contact.

 6. Reservoir permeability is low—generally less than 0.1 

millidarcy (mD).

 7. Reservoirs are overlain by normally pressured rocks 

containing both gas and water.

 8. Reservoirs contain primarily thermogenic gas.

 9. Migration distances are not thought to be great.

 10. Structural and stratigraphic traps are of minor importance.

 11. Reservoirs are commonly compartmentalized.

 12. Multiple fl uid phases contribute to seal development in 

reservoirs.

 13. The tops of basin-centered accumulations occur within 

a narrow range of thermal maturities, usually between 

vitrinite refl ectance values of 0.75 and 0.9 percent.

Many gas accumulations may have only some of the 

above characteristics, and classifi cation as a basin-centered 

accumulation may become highly subjective. 

Assessment of Basin-Centered 

Gas Accumulations

Assessment of basin-centered gas (and other continuoustype) accumulations is based on the concept that a continuous 

accumulation can be regarded as a collection of hydrocarbon-bearing cells. In the assessment unit, cells represent spatial subdivisions defi ned by the drainage area of wells. Cells 

may be productive, nonproductive, or untested. Geologic risk, 

expressed as assessment-unit probability, is assigned to each 

play. The number of untested cells in an assessment unit and 

the fraction of untested cells expected to become productive 

(success ratio) are estimated, and a probability distribution is 

defi ned for estimated ultimate recoveries (EUR’s) for those 

cells expected to become productive. The combination of 

geologic probability, success ratio, number of untested cells, 

and EUR probability distribution yields potential undiscovered 

resources for each assessment unit. Refer to Schmoker (1996) 

for a detailed discussion of continuous-type accumulations and 

their assessment.

In 1995, the U.S. Geological Survey defi ned 100 continuous-type oil and gas plays in sandstones, shales, chalks, and 

coals for all depth intervals. Of the 100 identifi ed plays, 73 

were gas plays. Eighty-six of the 100 identifi ed plays were 

quantitatively assessed. Estimates of undiscovered technically 

recoverable gas resources from all continuous-type plays—

excluding coal-bed gas plays—range from 219 trillion ft3

(TCF) (95th fractile) to 417 TCF (5th fractile), with a mean 

estimate of 308 TCF. Much of this resource is attributed to 

basin-centered gas plays. 

Four categories of continuous-type basin-centered gas 

plays can be identifi ed with respect to new data and perceptions since the U.S. Geological Survey 1995 National Petroleum Assessment: (1) plays that were assessed in 1995, but 

need to be updated because of new data; (2) plays that were 

identifi ed incorrectly as conventional or continuous in 1995; 

(3) plays that were identifi ed as continuous in 1995 but not 

assessed because of a lack of data; and (4) new continuoustype accumulations that were not identifi ed in 1995. Basincentered gas plays were not assessed or identifi ed in 1995 in 

many basins including the Wind River, Raton, Albuquerque, 

Bighorn, Crazy Mountains, and Hanna Basins of the Rocky 

Mountain region; the Anadarko and Arkoma Basins of the 

southern Midcontinent region; and the Colville Basin of northern Alaska.

Geologic Setting of Hanna Basin

The Hanna (fi g. 1) is a deep Laramide basin located 

northeast of the city of Rawlins in south-central Wyoming 

(Perry and Flores, 1997). The basin is fl anked by Precambriancored uplifts including the Shirley and Seminoe Mountains to 

the north, Simpson Ridge to the east, the Medicine Bow and 

Park Range uplifts to the south, and the Rawlins uplift to 

the west. Precambrian basement is about 30,000 to 40,000 

ft deep in the eastern part of the basin (Hansen, 1986). The 

Hanna Basin is considered a pull-apart basin; its tectonic history has been summarized by LeFebre (1988) and Perry and 

Flores (1997). Strike-slip and reverse faulting along the Shirley 

Mountain thrust occurred during Late Cretaceous and early 

Paleocene time (early Laramide), whereas uplift of the Medicine Bow Mountains and Rawlins uplift occurred during late 

Paleocene and Eocene time (late Laramide). 

Stratigraphic units in the Hanna Basin are shown in fi gure 

2. The Cambrian through Jurassic section is less than 2,500 

ft thick and includes the Cambrian Flathead Sandstone, Mississippian Madison Limestone, Pennsylvanian Casper Formation, Triassic Jelm and Chugwater Formations, and the Jurassic Nugget Sandstone and Sundance and Morrison Formations. The Lower and lower Upper Cretaceous fl uvial and 

marine section includes the Cloverly Formation, Thermopolis 

Shale, Muddy Sandstone, and Mowry Shale. Upper Cretaceous 

marine and deltaic units include the Frontier Formation, Niobrara Formation, Steele Shale, Mesaverde Group, Lewis Shale, 

Fox Hills Sandstone, and Medicine Bow Formation. Fluvial 

and deltaic sediments of the Tertiary Hanna and Cretaceous 

Potential for Deep Basin-Centered Gas Accumulation in Hanna Basin, Wyoming 3
Figure 2. Stratigraphic chart showing names and average thicknesses of stratigraphic units recognized in the Hanna 

Basin, Wyoming. Modifi ed from Kaplan and Skeen (1985, p. 224). 

STRATIGRAPHIC UNITS IN THE HANNA BASIN

PERIOD/EPOCH STRATIGRAPHIC UNIT Approximate

Thickness (feet)

EOCENE

Ferris Formation

Hanna

Formation

Medicine Bow Formation

Fox Hills Sandstone

Lewis Shale

Mesaverde Group

Steele Shale

Niobrara Formation

Frontier Formation

Mowry Shale

Muddy Sandstone

Thermopolis Shale

Cloverly Formation

Morrison Formation

Sundance Formation

Nugget Sandstone

Jelm and Chugwater Formations

Goose Egg Formation

Casper Formation

Madison Limestone

Flathead Sandstone

granitic and metamorphic

basement

PALEOCENE

LATE

EARLY

CRETACEOUS TERTIARY (part)

19,000

6,000

2,100

2,600

3,000

1,200

800

200

60

80

200

375

300

700

400

400

300

500

65

JURASSIC

TRIASSIC

PERMIAN

PENNSYLVANIAN

MISSISSIPPIAN

CAMBRIAN

PRECAMBRIAN

?

4 Geologic Studies of Basin-Centered Gas Systems
and Tertiary Ferris Formations fi lled the basin as it subsided 

rapidly during Laramide uplift and are almost 19,000 ft thick 

in the basin center (Hansen, 1986). Several thousand feet of 

sediment have probably been removed by erosion from the 

basin since Miocene time, based on the presence of regional 

unconformities.

Exploratory Drilling

The Hanna Basin has been lightly explored for oil and 

gas, including coal-bed methane. The lack of exploration is 

due in part to the “checkerboard” lease position in which 

alternate sections have been held by the Union Pacifi c Railroad 

Corporation (Stone, 1966). Most of the early exploratory wells 

tested fault-fold structures along the fl anks of the basin (Johnson and Flores, 1998), and only a few deep wells have been 

drilled in the basin center. A few minor oil and gas fi elds 

have been discovered, including the Rock River, Allen Lake, 

Big Medicine Bow, Cedar Ridge, Chapman Draw, Melton, and 

Simpson Ridge fi elds (Mapco, 1999). Figure 3 shows lines 

of cross section and well locations for 29 representative exploration wells. The Denver Earth Resources Library, Denver, 

Colo., provided much of the data used here, including geologic 

maps, well logs, and scout cards. 

Evidence for Normal Pressures

Drilling-mud-weight, drill-stem-test, bottom-holetemperature (table 1), and vitrinite refl ectance data were examined for evidence of abnormal pressures that might indicate the 

presence of a basin-centered gas accumulation. Figures 4 and 5 

illustrate our interpretations of the approximate boundaries of 

normal and abnormal pressure zones in the Hanna Basin based 

on the limited well data available. 

The Cambrian through Jurassic stratigraphic section is 

interpreted to be regionally water saturated and normally pressured largely due to the absence of signifi cant hydrocarbongenerating source rocks. Several exploratory wells recovered 

water with near-normal pressure gradients during tests of 

the Jurassic Morrison and Pennsylvanian Tensleep Formations 

(fi g. 3, symbol CKS). Some oil shows were encountered in the 

Triassic Chugwater Formation and Jurassic Nugget Sandstone. 

These shows are probably due to early (pre-Laramide) migration of oil generated from the Permian Phosphoria Formation 

in western Wyoming. Laramide tectonic movements disrupted 

and breached many of the pre-Cretaceous oil traps, and noncommercial oil shows are common in the Paleozoic section. 

Water-producing Paleozoic through Tertiary reservoirs 

with near-normal pressure gradients (0.40 to 0.47 psi/ft) have 

been encountered at depths of less than 8,000 ft along the 

western, southern, and southeastern margins of the basin, and 

at depths of less than 12,500 ft in the basin center (table 1). 

The Frontier-Cloverly section was tested in three wells on 

the west fl ank of the basin (table 1, symbols MHS, CKS, 

and SFM), and shut-in pressures from drill-stem tests indicate 

near-normal pressure gradients ranging from 0.35 to 0.47 

psi/ft in this part of the basin. Several tests of the Mesaverde 

Group and Ferris Formation (table 1) also recovered water 

with near-normal pressure gradients. One of the deepest wells 

in the basin, the Forgotson-Nortex HBJVU No. 1-25 well 

(fi g. 3, FGN) reached a total depth of 15,322 ft in the lower 

Mesaverde Group using only 8.9 pounds per gallon (ppg) mud. 

Overpressures were not reported in this well. Drill-stem tests 

at 12,500 and 10,200 ft recovered water, and shut-in pressures 

indicate normal pressure gradients of 0.41 psi/ft. 

These results indicate extensive water-saturation at depths 

less than 12,500 ft in the basin. The normally pressured zone 

(fi gs. 4 and 5) is interpreted to be an envelope extending from 

outcrops on the basin margins through the basin center at 

shallow depths, and from outcrops through the basin bottom 

within the Cambrian- through Jurassic-age section. Major fault 

zones and outcrops surrounding the basin may have allowed 

extensive water invasion and (or) gas leakage.

Evidence for Subnormal Pressures

Zones of subnormal pressure (±0.3 psi/ft) have been 

found below water-saturated, normal-pressure zones and above 

central overpressure zones in several other basin-centered gas 

accumulations in the Rocky Mountain region (Meissner, 2000; 

Wilson and others, 1998). The subnormally pressured zones 

are generally interpreted to have been previously overpressured during peak gas expulsion, but loss of gas due to cooling, erosional unloading, and escape along fractures and 

fault zones has allowed pressures to decline to subnormal 

levels. Identifi cation of subnormally pressured zones in exploration wells is often diffi cult because water-based drilling mud 

often overbalances the low-pressure gas reservoirs. Gas shows 

may be suppressed (Wilson and others, 1998) and formation 

damage may have occurred due to mud invasion. Drill-stem 

tests may be ineffective or inconclusive in the damaged intervals. Low or fl at mud-log gas readings, problems with lost 

circulation, and drill-stem tests that recovered gassy mud with 

low shut-in pressures are often subtle clues indicating subnormally pressured zones. 

Subnormal pressures have been interpreted in the central 

part of the Hanna Basin (fi gs. 4 and 5) at depths ranging 

from 10,000 to 25,000 ft based on indirect evidence including 

lost circulation reported at 11,800 ft in the Amoco Seminoe 

Reservoir No. 1 well, where 8.8 ppg mud was used. This mud 

may have overbalanced a low-pressure zone (fi g. 3, ASR). 

Temperatures high enough for gas generation have been 

Text continues on p. 10

Potential for Deep Basin-Centered Gas Accumulation in Hanna Basin, Wyoming 5
Figure 3. Map showing structural elements, outcrop patterns, locations of cross sections (fi gs. 4 and 5), and locations of 29 representative exploration wells 

Wyoming. Modifi ed from Hansen (1986, p. 485) and MAPCO (1999, maps WA5 and WA6). Three-letter code identifi es well name as shown in table 1.

R. 86 W. R. 85 W. R. 84 W. R. 83 W. R. 82 W. R. 81 W. R. 80 W. 

0 

MILES

HANNA BASIN

A

A′

B

B′

CKS

MHS

DLD

FOM

FMC ASR

SFM

COG

BCC

OOU

BCF AAF

ASM

TWF

FPC

HPC TCF

CSR

CRA

CLU

IXU

THC

FOD

ERH

BKH

FGN

SHIRLEY MOUNTAINS

SEMINOE MOUNTAINS

LEWIS SHALE

MEDICINE BOW FORMATION

FERRIS FORMATION

HANNA FORMATION

MEDICINE BOW FOR MATIO

N

HANNA

FORMATION

F

E

RRIS

FORMATION

LEWIS

SHALESIMPSON RIDGE
? Precambrian basement

FGN THC FOD 

9.5

9.5

8.9

0.410.41

204 F

0.38

?

?

A 

10

-10-20

SEA

LEVEL

-30-40

Kd

J - C Kle

Kmv

Ks

Kn

Kf-Km

Kmb

TKf

Th

TKf

Kmb

DEPTH, IN THOUSANDS OF FEET

0.41 = Gradient (psi/ft)9.5 = Mud weight (ppg)204 ̊F = Bottom hole temperature ( ̊F)

Normally pressured intervalSubnormally pressured intervalOverpressured interval

Gas-producing wellDry hole

Figure 4. Cross section A-A’ showing generalized structure of the Hanna Basin and approximate boundaries of the normal, subnormal, and overpressure zo

interpretations of vitrinite refl ectance measurements and well data. See fi gure 3 for location of cross section. See table 1 for explanation of codes used to iHanna Formation; TKf, Tertiary-Cretaceous Ferris Formation; Kmb, Cretaceous Medicine Bow Formation; Kle, Cretaceous Lewis Shale; Kmv, Cretaceous MeCretaceous Steele Shale; Kn, Cretaceous Niobrara Formation; Kf, Cretaceous Frontier Formation; Km, Mowry Shale; J-e, Jurassic through Cambrian rocks. 
J - C

12.1

12.9

1.2%

9.3

Kle

Kf-KmKn

?

DEPTH, IN THOUSANDS OF FEET Kd

B 

SW 

10

SEA

LEVEL

-10-20-30-40

COG SFM OOU BCC THC BKH

0.43

0.47

0.31

12.1

11.9

12.9 9.5

0.37

1.2%

9.3

0.42

Precambrian basement 

Th TKf

Kmb

Kle

Kf-KmKn

? Vertical scale: 1" = 10,000 ft

Horizontal scale: 1" = 20,000 ft

1.2% = Virtrinite reflectance

DEPTH, IN THOUSANDS OF FEET

0.42 = Pressure gradient (psi/ft)

9.3 = Mud weight (ppg)

Normally pressured intervalSubnormally pressured intervalOverpressured interval

Gas-producing wellDry hole

Kd

Figure 5. Cross section B-B’ showing generalized structure of the Hanna Basin and interpreted boundaries of normal, subnormal, and overpressure zones, 

vitrinite refl ectance measurements and well data. See fi gure 3 for location of cross section. See table 1 for explanation of codes used to identify wells. Th, TTKf, Tertiary-Cretaceous Ferris Formation; Kmb, Cretaceous Medicine Bow Formation; Kle, Cretaceous Lewis Shale; Kmv, Cretaceous Mesaverde Group; KsKn, Cretaceous Niobrara Formation; Kf, Cretaceous Frontier Formation; Km, Mowry Shale; J-e, Jurassic through Cambrian rocks. 
Amoco Alkali Flat Unit No. 1

Amoco Seminoe Reservoir 1Amoco St Mary's Unit No. 1Apache U.S.Golden No. 1Brinkerhoff Hanna Unit No. 1Byron Oil Cedar Ridge No. 1Byron Oil Cedar Ridge No. 1Chambers Klaenhammer No. 1Champlin UPRR 44-5 No. 1Coastal COGC-UPRR No. 1Colorado Oil No. 1 Gov'tContinental Simpson No. 1CRA CRA-UPRR No. 16-1Dillard Unit No. 41-22ERG HSR No. 34 RD-1Ferguson PCR 63X-30 No. 1FMC Sandstone Unit No. 1-13Forest Oil Dana Unit No. 1Forest Oil Miller No. 14-1Forgotson-Nortex HBS No. 1Humble Pass Creek No. 1INTEX UPRR No. 10-27Mobil F-14-4-G Murphy Haystack No. 1Ohio Oil Unit No. 1Sinclair Federal Melton No. 1True Oil Curry Federal No. 44True Oil Hanna Unit C No. 2Total Walcott Fed No. 1-20

Well Name

AAF

ASR

ASM

ACG

BKH

BCC

BCF

CKS

CPC

CLU

COG

CSRCRA

DLD

ERH

FPC

FMC

FOD

FOM

FGNHPC

IXU

MOFMHS

OOU

SFM

TCF

THC

TWF

Symbol

1517

9

22

3

152216

5

2134171622

2

3013

4

14253327

4

142226342320

Sec.

22 N.23 N.21 N.22 N.23 N.22 N.22 N.23 N.21 N.22 N.22 N.21 N.22 N.23 N.24 N.21 N.23 N.22 N.23 N.24 N.21 N.24 N.21 N.23 N.22 N.22 N.21 N.23 N.21 N.

T.

84 W.84 W.84 W.79 W.82 W.85 W.85 W.86 W.79 W.80 W.86 W.80 W.80 W.85 W.83 W.82 W.85 W.82 W.85 W.84 W.82 W.81 W.79 W.86 W.85 W.86 W.81 W.83 W.84 W.

R.

19781977197419691972198219801976198219901958195719681963197819661980195519561982196919601958195119541961197019731985

Year

9,870

12,10015,553

9,238

12,50010,67210,302

9,240

11,35110,050

4,842

10,514

5,0134,9245,1267,2777,078

10,706

7,910

15,32216,850

9,587

12,623

7,472

13,177

3,2156,9508,6235,172

T D

feet

Ks

Kmv

Mm

Jm

TKf

KnKn

Jm

KfKf

KmvKmvKmvKmv

TKf

Kmv

Ks

Kmb

JmJmJmJm

TKf

Kn

Fm at TD

9.69.18.89.0

10.010.9

9.29.29.3

12.9

8.9

10.910.5

9.29.9

12.010.011.511.4

9.29.29.39.6

10.0

9.69.59.59.88.79.08.99.89.09.29.8

10.110.0

9.79.89.7

10.912.111.910.611.0

9.59.2

Mud

ppg

9,8059,791

12,097

5,968

11,26614,850

9,199

10,70012,49810,648

6,185

10,288

9,240

11,338

6,3007,200

10,050

9,870

10,513

4,8904,5485,1263,6657,2777,078

10,706

6,3697,906

10,02312,68615,190

6,908

16,850

6,0959,497

10,32111,246

6,8417,4666,755

10,12611,79612,213

6,1246,9508,6235,176

Depth

feet

165160178102181217158160109136140185164110157

94

100114111127154127176204110240115143148172147160184125152

BHT

deg F

2,2732,695

241

1,5882,6852,5383,8091,5232,0821,8171,9682,953

450

4,2325,1644,9314,9622,0001,160

962

1,4853,146

509

DST SIP

psi

8,8506,3509,080

10,350

6,2207,1509,1804,8254,9904,4504,7407,0007,060

10,30012,50013,20011,070

6,4007,3302,2453,1508,3904,400

Depth

feet

0.260.42

very lowvery low

0.430.370.410.310.420.410.420.42

very low

0.410.410.370.450.31

very low

0.430.470.38

very low

Gradient

psi/ft

No tests. Some NDXO effects. Normal pressure ??DST recovered 1,900 feet of sgcm water. Normal pressuCored 11,720-780 feet, lost circulation. Subnormal pressPerforated Niobrara at 10,470-10,560 feet. Abandoned.Probably overpressured in Niobrara & Frontier.No tests reported. Niobrara may be subnormally pressuNormal pressure gradient at 6,350 feet? Few shows. Ro < 0.7% above 10,000 feet.Dpor = 5-13%. Ro = 1.2% near TD. Subnormal pressure12.9 ppg mud, overpressure? Perforated Shannon Ss. (10.9 ppg mud. Perforated Niobrara and Shannon Ss. (StDST Frontier recovered gassy mud. Probably normal preDST Dakota (Cloverly) recovered 3,750 feet of water. NDST Tensleep recovered 7,100 feet of sulfur water. NormFlowed gas and water from Morrison. Normal pressure.Gas kicks, 12 ppg mud. Overpressured Niobrara FormatSteep dips, stuck pipe, two sidetracks. Abandoned.DST Tensleep rec. 100 feet of water. Normal pressure?Gas shows, mud weight = 11.4 ppg in Niobrara. Fault zoNiobrara section is probably overpressured. AbandonedDST Kmv recovered 316 feet of water. Normal pressureDST Kmv recovered 2,694 feet of water. Normal pressuDST Kmv recovered 4,355 feet of water. Normal pressuLow BHT. Probably normal pressure. No tests. No formation tops listed on card.DST Kmv rec. 6,173 feet of water. Normal pressure.Ferris is probably normally pressured.DST Kmv recovered 60 feet gas-cut mud. Subnormal prDST Kl rec. 3,712 feet water. Normal pressure.DST Kmv recovered 11,061 feet gas, water and mud. MKmv is probably normal pressure, subnormal near TD?DST Ksh flowed gas 44 MCFD, rec. 1,675 feet of oilKmv is probably normal pr. Niobrara may be subnormal. Gas show noted at 9,220 feet. No cores or tests, TD in Medicine Bow. Abandoned.10.1 ppg mud through Niobrara-Frontier. Several cores. DST Jn flowed gas, subnormal & normal pressures (?)DST Frontier recovered 50 feet gassy mud. Subnormal DST Dakota (Cloverly) recovered 1,440 feet of water, tra

Gas kick, DST flowed gas from Niobrara. Abandoned. Niobrara-Frontier Shale is probably overpressured.DST Frontier recovered 1,316 feet of water. Normal presDST in Dakota recovered gas and mud. Normal pressurNo tests. Niobrara-Frontier may be overpressured.High mud weight. DST recovered 560 feet muddy water. Probably normal Niobrara is probably subnormally pressured.

Comments

Table 1. Mud weights, bottom-hole temperatures and drill-stem test results for 29 representative exploration wells in the Hanna Basin, Wyoming, based on w

available at the Denver Earth Resources Library, Denver, Colo.

[Fm., formation; ppg, pounds per gallon; NDXO, neutron-density crossover; Dpor, density porosity; pr., pressure; TD, total depth of well; MCFD, thousand ft3 of gas per day; BHT, bo

pounds per square inch per foot; Ro, vitrinite refl ectance in percent; DST SIP, standard drill-stem test shut-in pressure; sgcm, slight gas cut mud; Ss, sandstone; Mm, MississippiaPennsylvanian Tensleep Formation; PPa, Pennsylvanian Amsden Formation; Jm, Jurassic Morrison Formation; Jn, Jurassic Nugget Sandstone; Kn, Cretaceous Niobrara FormatioKmv, Cretaceous Mesaverde Group; TKf, Tertiary-Cretaceous Ferris Formation]
reported from deep wells in the Hanna Basin. A temperature 

of 204°F was reported from a drill-stem test at 11,871 to 

15,322 ft in the Forgotson Seminoe Unit No. 1-25 (fi g. 3, 

FGN), indicating that temperatures are high enough for a 

basin-centered gas accumulation to exist. However, the drilling 

mud used in this well was only 8.9 to 9.0 ppg and overpressures were not encountered. These rocks might have been 

overpressured previously, but pressures may have declined to 

subnormal levels due to cooling and leakage of gas through 

fractures and outcrops updip to the northwest. Subnormal pressure is interpreted below 15,000 ft in this well (fi g. 4). 

Measured vitrinite refl ectance values in the Brinkerhoff 

Hanna Basin Unit No. 1 well (fi g. 3, BKH) vary from less than 

0.7 percent above 10,000 ft to 1.23 percent at 12,845 ft (Law, 

1984; Perry and Flores, 1997). Source rocks are thermally 

mature below 10,000 ft; however, normal mud weights were 

used in this well (9.2 ppg at 10,700 ft and 9.3 ppg at 12,498 

ft) indicating that overpressuring probably does not exist. A 

drill-stem test at 10,302 to 10,380 ft recovered gassy mud with 

a very low shut-in pressure (1,588 psi), indicating a tight, damaged, or low-pressure zone. Subnormal pressures are highly 

likely in this depth range. 

Three wells on the south and west margins of the basin 

(fi g. 3, MHS, TWF, and HPC) tested the Niobrara and Frontier 

Formations and recovered gas and (or) oil shows with very 

low shut-in pressures. The top of the subnormally pressured 

zone (fi gs. 4 and 5) is interpreted to cut across stratigraphic 

boundaries near the basin margins, and these wells may have 

penetrated the subnormally pressured zone where it cuts across 

the Niobrara-Frontier-Mowry section.

Evidence for Overpressured Zones

Drilling-mud-weight data (table 1) indicate that the Niobrara-Frontier-Mowry section may be moderately overpressured at depths below 10,000 ft along the western and southern 

margins of the basin. Gas shows and mud weights as high as 

12.9 ppg were reported at the Byron Cedar Ridge No. 1 well 

(fi g. 3, BCC)—reported mud weight was as high as 12.1 ppg 

at the Ohio Oil Unit No. 1 (fi g. 3, OOU) while drilling through 

the Niobrara section. Several other wells along this trend also 

encountered gas shows and gas kicks and used 10- to 12-ppg 

drilling mud through the Niobrara-Frontier interval (fi g. 3, 

BCF, ASM, TCF, CSR, MOF, and CLU). Mud-log descriptions indicate that this interval consists of calcareous shale 

beds and thin, low-permeability sandstones with poor reservoir 

quality. Gas kicks may have occurred in locally fractured 

zones with unusually high permeability. Several completion 

attempts using perforations and artifi cial fracture stimulation 

in cased holes were unsuccessful at establishing commercial 

production from the Niobrara Formation in the area. This 

gas-saturated, fractured shale reservoir may require special 

completion methods. 

Overpressuring was not encountered in any of the deep 

exploratory wells drilled near the center of the basin. The 

Forgotson-Nortex No. 1-25 well (FGN) reached a total depth 

of 15,322 ft in the lower Mesaverde Group using 8.9 ppg 

mud. This well may have penetrated subnormally pressured 

rocks near total depth. The limited well data available show no 

evidence of gas-charged overpressures within the Upper Cretaceous Mesaverde Group, Lewis Shale, Medicine Bow Formation, or in the Cretaceous and Tertiary Ferris or Tertiary Hanna 

Formations in the basin center. Based on these results, the top 

of the overpressure zone is interpreted to extend downdip from 

the Byron, Ohio, and Continental wells (fi g. 3, BCC, OOU, 

CSR) and across the center of the basin at depths of 18,000 to 

25,000 ft (fi gs. 4 and 5). This deep overpressured zone appears 

to have a diameter of approximately 20 to 25 miles.

Basin modeling indicates that source rocks in the Niobrara-Frontier-Mowry interval are thermally mature in the deep 

basin and have generated large volumes of oil and gas (Beirei 

and Surdam, 1986; Beirei, 1987). However, transformation 

models suggest that hydrocarbon generation may have peaked 

during Paleocene and Eocene time due to the rapid deposition 

of many thousands of feet of the Ferris and Hanna Formations 

within a 15 million year time span. Gas generation may have 

slowed or ceased during post-Eocene time as a result of uplift 

and erosion, and the overpressure zone may have receded from 

its maximum extent. 

Discussion: Basin-Centered Gas 

in the Deep Hanna Basin?

The unusually high mud weights used in wells penetrating the Niobrara-Frontier-Mowry section along the southern 

and western margins of the basin indicate some potential for 

a basin-centered gas accumulation. Similar overpressures in 

the Niobrara-Frontier-Mowry section have been described in 

the Piceance Basin in western Colorado by Wilson and others 

(1998), in the Washakie Basin of south-central Wyoming by 

Kristinik and Lorenz (2000), and in the Wind River Basin 

by Johnson and others (1996). The interpreted boundaries of 

the normally pressured and subnormally pressured zones in 

the Hanna Basin fi t the patterns of typical basin-centered gas 

accumulations. Basin modeling by Beirei (1987) suggests that 

the Niobrara-Frontier-Mowry section has probably reached (or 

even exceeded) the gas-generation window in the deep basin. 

This part of the section may have been extensively dewatered 

and charged with gas. 

The Hanna Basin is relatively small compared to other 

basins in the Greater Green River Basin of southwestern Wyoming. It is surrounded by basement uplifts along fault zones 

that have thousands of feet of displacement. Gas may have 

escaped via several major fault and fracture zones or through 

outcrops along the margins of the basin. Extensive water invasion may be occurring along the major fault zones, and formerly abnormal pressures may be reverting to normal pres10 Geologic Studies of Basin-Centered Gas Systems
sures. The interpreted pressure boundaries (fi gs. 4 and 5) suggest extensive normal and subnormal pressures and a relatively 

small (and very deep) overpressured zone. 

No wells have been drilled deep enough to validate the 

concept of a central overpressured zone. Core analyses and 

measured permeability data were not found in the well data 

collection. Well-log analyses by Schlumberger (noted on the 

Brinkerhoff No. 1 density log) indicate that sandstone porosities range from 5 to 13 percent at depths of 10,600 to 12,400 

ft. These values are typical for Cretaceous and Tertiary tight 

gas sands in Wyoming. Porosity and permeability ranges for 

thin sandstones in the Frontier Formation are unknown but 

probably resemble those reported by Kristinik and Lorenz 

(2000) for Frontier sandstones at the Sidewinder 4-H well 

in the Washakie Basin (9 to 12 percent, 0.04 to 0.001 mD). The 

fractured Niobrara, Frontier, and Mowry reservoirs are technically challenging reservoirs that have not yet been commercially productive in other known basin-centered gas accumulations. New technologies may be needed to produce gas from 

these deep, thermally mature, Cretaceous, shale-rich reservoirs 

in the Hanna and other Rocky Mountain basins. 

Conclusions

Data for deep wells in the Hanna Basin show evidence 

for a shallow, normally pressured section, subnormal pressures 

at intermediate depths, and a gas-charged overpressured zone 

within the Niobrara-Frontier-Mowry section along the southern and western margins of the basin. This overpressured zone 

may be part of a larger, continuous gas accumulation that 

extends through the basin center at depths below 18,000 to 

25,000 ft. No evidence was found for overpressuring in wells 

penetrating the Upper Cretaceous Mesaverde Group and Lewis 

and Medicine Bow Formations, and Cretaceous and Tertiary 

Ferris and Tertiary Hanna Formations, at depths of less than 

15,000 ft. 

Basin modeling by Beirei (1987) indicates that Cretaceous-age source rocks in the deep basin reached thermal 

maturity relatively quickly due to rapid burial below 18,000 ft 

of the Paleocene Hanna and Upper Cretaceous and Paleocene 

Ferris Formations. Gas generation probably peaked in Paleocene and Eocene time and may have slowed or ceased during 

late Tertiary time. 

Only a few deep wells have been drilled in the center 

of the Hanna Basin, and the deepest well drilled to date (Forgotson-Nortex No. 1, 15,322 ft total depth) did not encounter 

overpressured zones. An overpressured zone may have receded 

from its maximum extent due to erosional unroofi ng, cooling, 

and gas leakage via faults, fractures, and outcrops. The present 

diameter of a deep overpressured compartment may be only 20 

to 25 miles. A relatively thick zone of subnormal pressure has 

been interpreted where previously overpressured strata have 

lost gas pressure. 

The interpretations proposed in this report remain speculative due to the limited data available for the Hanna Basin. 

However, available data suggest a pattern favoring the presence of a basin-centered gas accumulation similar to accumulations in other basins of the Rocky Mountain region (Wilson 

and others, 1998).

References Cited

Beirei, M.A., 1987, Hydrocarbon maturation, source rock potential, and 

thermal evolution of Late Cretaceous and early Tertiary rocks of 

Hanna Basin, southeastern Wyoming: Laramie, Wyo., University of 

Wyoming M.S. thesis, 130 p.

Beirei, M.A., and Surdam, R.C., 1986, Hydrocarbon maturation, source 

rock potential, and thermal evolution of Late Cretaceous and early 

Tertiary rocks of the Hanna Basin [abs.], southeastern Wyoming: 

American Association of Petroleum Geologists Bulletin, v. 70, no. 

8, p. 1031.

Gautier, D.L., Dolton, G.L., Takahashi, K.I., and Varnes, K.L., eds., 1996, 

1995 National assessment of United States oil and gas resources—

Results, methodology, and supporting data: U.S. Geological Survey 

Digital Data Series 30, release 2. 

Hansen, D.E., 1986, Laramide tectonics and deposition of the Ferris 

and Hanna Formations, south-central Wyoming: American Association of Petroleum Geologists Memoir 41, p. 481–495.

Johnson, R.C., Finn, T.M., Keefer, W.R., and Szmajter, R.J., 1996, Geology of Upper Cretaceous and Paleocene gas-bearing rocks: Wind 

River Basin, Wyoming: U.S. Geological Survey Open-File Report 

96-090, 120 p.

Johnson, R.C., and Flores, R.M., 1998, Developmental geology of coalbed methane from shallow to deep in Rocky Mountain basins and 

in Cook Inlet–Matanuska Basin, Alaska, U.S.A and Canada: International Journal of Coal Geology, no. 35, p. 241–282.

Kaplan, S.S., and Skeen, R. C., 1985, North-south regional seismic 

profi le of the Hanna Basin, Wyoming, in Gries, R.R., and Dyer, R.C., 

eds., Seismic Exploration of the Rocky Mountain Region: Rocky 

Mountain Association of Geologists and Denver Geophysical Society, p. 219–224.

Krystinik, L.F., and Lorenz, J.C., 2000, Do you want to hear the good 

news or the bad news? New perspectives on basin-centered gas 

from horizontal drilling, Frontier Formation, SW Wyoming, in Rocky 

Mountain Association of Geologists 2000 Basin Center Gas Symposium Abstracts, October 6, 2000: Rocky Mountain Association of 

Geologists. 

Law, B.E., 1984, Relationships of source rock, thermal maturity and 

overpressuring to gas generation and occurrence of low-permeability Upper Cretaceous and lower Tertiary rocks, Greater Green 

River Basin, Wyoming, Colorado, and Utah, in Woodward, J., 

Meissner, F.F. and Clayton, J.L, eds., Hydrocarbon Source Rocks of 

the Greater Rocky Mountain Region: Rocky Mountain Association 

of Geologists Guidebook, p. 401–433.

LeFebre, G.B., 1988, Tectonic evolution of Hanna Basin, Wyoming—

Laramide block rotation in the Rocky Mountain foreland: Laramie, 

Wyo., University of Wyoming Ph.D. dissertation, 240 p.

Potential for Deep Basin-Centered Gas Accumulation in Hanna Basin, Wyoming 11
MAPCO, Inc., Tulsa, Okla. 74119, 1999, Maps WA-5 and WA-6. 

Meissner, F.F., 2000, Causes of anomalous deep basin fl uid pressures 

in Rocky Mountain basins and their relation to regional gas accumulation, in Rocky Mountain Association of Geologists 2000 Basin 

Center Gas Symposium Abstracts, October 6, 2000: Rocky Mountain 

Association of Geologists. 

Perry, W.J., and Flores, R.W., 1997, Sequential Laramide deformation 

and Paleocene depositional patterns in deep gas-prone basins of 

the Rocky Mountain region, in Dyman, T.S., Rice, D.D., and Westcott, P.A., eds., Geologic Controls of Deep Natural Resources in the 

United States: U.S. Geological Survey Bulletin 2146, p. 49–59.

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of United States oil and gas resources—Results, methodology, and 

supporting data: U.S. Geological Survey Digital Data Series 30, 

release 2.

Stone, D.S., 1966, Evaluation of the Hanna Basin: The Mountain Geologist, v. 3, no. 2, p. 55–73. 

U.S. Geological Survey National Oil and Gas Resource Assessment 

Team, 1995, 1995 national assessment of United States oil and gas 

resources: U.S. Geological Survey Circular 1118, 20 p.

Wilson, M.S., Gunneson, B.G., Peterson, K., Honore, R. and Laughland, 

M.M., 1998, Abnormal pressures encountered in a deep wildcat 

well, southern Piceance Basin, Colorado, in Law, B.E., Ulmishek, 

G.E., and Slavin, V.I., eds., Abnormal Pressures in Hydrocarbon 

Environments: American Association of Petroleum Geologists 

Memoir 70, p. 195–214.

Manuscript approved for publication June 14, 2001

Published in the Central Region, Denver, Colorado

Editing, layout, photocomposition—Richard W. Scott, Jr.

Cover design—Carol A. Quesenberry

Graphics by the authors

12 Geologic Studies of Basin-Centered Gas Systems